Red square when preview an image MPB16

1 Restart your Mac.

    Launch Safari holding the shift key down and test.

2 Startup in  safe mode to try to isolate the cause of the issue.

Select your account and login.

Test.

   To leave Safe Mode, click Apple logo   in the menu bar and choose “Restart”.

   https://support.apple.com/en-us/HT201262

   Test.

I think you'll see that the big red blob disappears when you switch to develop as you mentioned. BUT, I don't think this has anything to do with Lightroom at all, because I can generate the same problem in a folder of JPEGs in finder. By using the space-bar preview, images will randomly come up as big reg squares, just like this. And interestingly, even for the ones that come up correctly, there is an instant where the image is red before it switches over to being correct. All this advice about resetting pram, or messing with the GPU -- I haven't seen a single one of those explain in the slightest what that might accomplish. So for that reason, I think they are just taking wild guesses, without having any idea of where this is coming from. One thing seems consistent -- it's after an installation of Big Sur. So I'm thinking the fault lies with Apple, but they're not fessing up yet, or haven't devoted any resources to fixing this annoyance. How about it APPLE?

-- Matt Temple

Can you drag it?

Try these hardware resets, if these don't help, please contact Apple support.

Reset PRAM:  Reset NVRAM or PRAM on your Mac - Apple Support

Reset  SMC:   How to reset the SMC of your Mac - Apple Support   Choose the appropriate method.

Contact Apple Support

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Did this happen after trying to take a screenshot?

You can also try force quitting Finder.

Quit all applications force quit Finder app and choose Relaunch Finder.

https://support.apple.com/en-us/HT201276

Hi!

I am having tha same problem with my MBP 15" 2018.

After installing Big Sur the red squares appear every time trying to quick look something, it doesn't matter what kind of file I'm trying, it just appears briefly as when it is "loading" the preview. I discovered that it has to do with the external monitor, or the use of the discrete GPU, as when I don't have it plugged the preview displays fine.

I tried NVRAM reset, no success.

I tried on Safe Boot and found some issues:

• The previews have an intermediate step when opening in which they freeze midway and look blurry, and some kind of weird transparent frame around them, after some seconds, they finally open, and the fan starts speeding up, and when trying to close the preview, it freezes, and sometimes even brings me to the log in screen again. When I connect the secondary monitor and try the same workaround it kind of works better than without it.
• Tried opening Safari and scrolling down was super laggy. When connecting the external monitor, it again got a bit better.
• The red squares were replaced by this laggy, weird versions of previews.
• Back from safe boot, the red squares are still there.
• In Activity Monitor the processes CVM Complier, Window server and QuickLookUIService (Finder) were peaking in both CPU and Memory use.
• Both starting in Safe boot and regularly, I noticed for a brief instant a pop up window with the yellow warning exclamation sign, but it is too brief to even try to take a screen shot, so I have no idea what is causing it or what it is saying.

Maybe this report helps to find out what the problem might be.

Any ideas to try out are welcome!

i have the same issue after update

Probably it is one of "those" things known as software glitches.